1. Field of the Invention
The present invention relates to an image forming apparatus with an image reading apparatus, and more particularly, to an image forming apparatus that suppresses noise radiating from an image reading apparatus.
2. Description of the Related Art
Image reading apparatuses are often connected to a printer or the like and used as one of the components of image forming apparatuses such as copiers or multi-function peripherals (MFPs), although they may be used standalone.
FIG. 7 is a top view of an image forming apparatus with an image reading apparatus, wherein components of a movable image reading apparatus 201 are mainly illustrated. In FIG. 7, the movable image reading apparatus 201 has a carriage 210, and the carriage 210 has a lamp 211, an image sensor 212, and an image sensor substrate 213.
The lamp 211 serves as a light source that emits light to an original. The emitted light is reflected by the original. Then, the reflected light is incident on the image sensor 212 through a lens or the like (not illustrated). The image sensor 212 includes a photoelectric conversion element such as a line CCD, and it is mounted on the image sensor substrate 213. The image sensor 212 is driven by an image sensor driving circuit 215 formed in the image sensor substrate 213 (see FIG. 8).
The carriage 210 is coupled to a timing belt 221. The timing belt 221 is rotatably supported by a driving pulley 222 and a driven pulley 223. The driving pulley 222 is connected via a gear or the like to a rotating shaft of a stepping motor 220. In addition, the carriage 210 is slidably fitted into a guide shaft 310 and slidably engaged with a guide rail 311. Further, the guide shaft 310 and the guide rail 311 are mounted on a side surface of the image reading apparatus 201 at a predetermined spaced-apart distance from, and in parallel to, a platen glass formed on the ceiling surface of the image reading apparatus 201.
According to this configuration, the carriage 210 is moved in right and left directions in FIG. 7 reciprocally along the bottom surface of the platen glass by forward/reverse rotations of the stepping motor 220, by which the original (information on the original) is exposed and scanned accordingly. In this case, a position sensor 224 senses that the carriage 210 is moved to its home position. Upon sensing, the rotational direction of the stepping motor 220 is reversed.
FIG. 8 is a block diagram schematically showing a configuration of the control system of the movable image reading apparatus 201. The carriage 210 has not only the above-mentioned lamp 211, image sensor 212, and image sensor substrate 213, but also a lamp inverter 217 to turn on the lamp 211. In addition, an A/D conversion circuit 216 and an image sensor driving circuit 215 (a drive circuit unit) are mounted on the image sensor substrate 213 together with the image sensor 212. The image sensor 212 photoelectrically converts the image light associated with the original under a drive control of the image sensor driving circuit 215 and outputs as an analog image signal. The analog image signal is converted to a digital image signal by the A/D conversion circuit 216.
The digital image signal is transferred to an image processing substrate 102 (an image processing unit within the image reading apparatus) through a video cable 214. The video cable 214 includes a flexible flat cable (harness). The image processing substrate 102 is provided with a CPU 151 that controls the image reading apparatus 201, a RAM 153, and a ROM 152 that stores program for controlling the image reading apparatus 201. Between the image processing substrate 102 and the image sensor substrate 213, for example, signals are transmitted and received through the video cable 214. In this case, for example, the image processing substrate 102 supplies power from the power source and driving signals for the image sensor 212 to the image sensor substrate 213. On the other hand, the image sensor substrate 213 outputs, e.g., digitized image signals to the image processing substrate 102.
The image sensor driving circuit 215 supplies to the image sensor 212 a driving clock signal with a high frequency of on the order of 10 MHz to read a signal charge (an image signal) from the image sensor 212. Supplying such a high-frequency driving clock signal generates noise. The generated noise is radiated from the image sensor substrate 213 or the video cable 214. The radiated noise has a negative impact on, e.g., electric appliances.
Measures for alleviating such radiated noise from the above harness (the video cable 214) include a method using a flexible flat video cable with an electrostatic shield (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 02-308667). As described above, providing an electrostatic shield to the video cable 214 could reduce the radiated noise to some extent.
However, there has not been achieved sufficient reduction in such radiated noise, because feeble radiated noise leaked from the harness or the substrate provided with such an electrostatic shield is combined with the electrically instable conductive members in vicinity and those conductive members become antennas accordingly, so that radiation efficiency increases.
In the image forming apparatus with the above-mentioned image reading apparatus 201, the guide shaft 310 and the guide rail 311 of the image reading apparatus 201 may serve as the above-mentioned antennas.
That is, the guide shaft 310 and the guide rail 311 of the image reading apparatus 201 are earthed by conductive members such as earth lines or sheet metals with a side surface of the housing of the image reading apparatus 201 used as an earth path. The purpose of using a side surface of the housing of the image reading apparatus 201 as an earth path is to allow the carriage 210 to move successfully in an image reading operation. In addition, the housing of the image reading apparatus 201 and the housing of the image forming apparatus 101 are earthed at the rear-end portion of the side surface.
However, in such earth connection, as an earth path involves a large number of members, the length of the earth path becomes longer and contains impedance accordingly. As a result, this leads to electrical instability, particularly in higher frequency regions, and increases an intensity level of the radiated noise.
Further, in the above-mentioned earth connection, a GND (ground) loop R101 is formed by the guide shaft 310, the guide rail 311, and the conductive members through which the guide shaft 310 and the guide rail 311 are connected to each other (see FIG. 9). The GND loop R101 serves as a loop antenna so that the intensity level of radiated noise increases at a wavelength corresponding to N and 1/N times the loop length.
It is considered, as an approach for reducing the intensity level of radiated noise, to reduce the loop area or the loop current of a GND loop. However, the reduced loop area would lead to reduction in size of readable originals, etc., which would not be considered feasible. In addition, the reduced loop current would lead to decrease in quality of the read images or the like, which would not be considered feasible.